Ultrasound from a focused ultrasonic transducer can be used to selectively treat regions within the interior of the body. Ultrasonic waves are transmitted as high energy mechanical vibrations. These vibrations induce tissue heating as they are damped, and they can also lead to cavitation. Both tissue heating and cavitation can be used to destroy tissue in a clinical setting. However, heating tissue with ultrasound is easier to control than cavitation. Ultrasonic treatments can be used to ablate tissue and to kill regions of cancer cells selectively. This technique has been applied to the treatment of uterine fibroids, and has reduced the need for hysterectomy procedures.
To selectively treat tissue, a focused ultrasonic transducer can be used to focus the ultrasound on a particular treatment or target volume. The transducer is typically mounted within a medium, such as degassed water, that is able to transmit ultrasound. Actuators are then used to adjust the position of the ultrasonic transducer and thereby adjust the tissue region that is being treated.
Focused ultrasonic transducers also typically have multiple transducer elements, wherein the amplitude and/or phase of the transducer elements are controllable. In particular the phase of individual or groups of transducer elements is often controlled to control the location of the focus of the ultrasound. This enables the rapid adjustment location of the focus and the sequential sonication of different sonication points or locations. The tissue of a subject between the transducer and a sonication point is typically referred to as the near field region. The ultrasound travels through the near field region to the sonication volume. This intermediate tissue is also heated, although not as much as the sonication volume. When sonicating multiple sonication points the near field region of the different sonication points may overlap. Because a particular portion of the near field region may overlap it may be heated multiple times. To avoid overheating this overlapping near field region there may need to be delays between sonicating multiple sonication points.